This invention pertains to a silicon nitride (Si.sub.3 N.sub.4) ceramic body and a process for preparing the ceramic body.
Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, silicon nitride ceramics are resistant to chemical attack, particularly to oxidation. Because of these attributes, silicon nitride is useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines.
Failure of silicon nitride ceramics is generally associated with brittleness and flaws. The object therefore is to prepare a silicon nitride ceramic with high fracture toughness (K.sub.IC) and strength. Fracture strength is directly proportional to the fracture toughness and inversely proportional to the square root of the flaw size. High fracture toughness combined with small flaw size is therefore highly desirable, Mono-lithic silicon nitride, however, has a relatively low fracture toughness of about 5 MPa (m).sup.1/2.
In order to obtain a fully densified silicon nitride ceramic, a densification aid, such as magnesia, is almost always necessary. On sintering the densification aid typically forms a glassy grain boundary phase, which acts as a matrix into which the grains of crystalline silicon nitride are embedded. Disadvantageously, the glassy phase is responsible for a loss of strength and a low creep resistance in the ceramic at high temperatures. It is believed that this lowering results from a softening of the glass and a concomitant enhancement in subcritical crack growth and grain boundary sliding. In addition, the glassy phase is responsible for a lowering of oxidation resistance at high temperatures.
It is reported that the high temperature material properties of silicon nitride ceramics can be improved if the grain boundary phase is crystalline rather than glassy. For example, A. Tsuge, K. Nishida and M. Komatsu disclose in the Journal of the American Ceramic Society, 58, (1975) 323-326, a hot-pressed silicon nitride ceramic containing a crystalline grain boundary phase identified as Si.sub.3 N.sub.4.Y.sub.2 O.sub.3 prepared by pre-sintering and then hot-pressing a powder predominantly of silicon nitride and yttria. The ceramic is disclosed to have improved high temperature strength when compared to a silicon nitride ceramic having a glassy grain boundary phase of yttria. Disadvantageously, the pre-sintered and hot-pressed ceramic does not have sufficient strength and toughness to meet current commercial standards.
U.S. Pat. No. 4,920,085 discloses a silicon nitride sintered body comprising .beta.-Si.sub.3 N.sub.4 and further comprising a crystalline grain boundary phase in an amount less than 90 weight percent of the total grain boundary phases, the balance being a glassy composition. The crystalline phase contains one or more of the stoichiometric compositions M.sub.4 Si.sub.2 O.sub.7 N.sub.2, M.sub.10 Si.sub.2 O.sub.23 N.sub.4 or MSiO.sub.2 N, wherein M is selected from the group consisting of scandium, terbium, erbium, holmium, and dysprosium. Disadvantageously, this sintered body possesses a considerable amount of glassy grain boundary phase which lowers strength and creep resistance.
It would be very desirable to have a silicon nitride ceramic of high fracture toughness, high fracture strength, and high oxidation resistance at high temperatures. Moreover, it would be highly desirable to have a process which would be reproducible, inexpensive, and amenable to industrial scale-up for preparing such a tough and strong silicon nitride ceramic.